This invention relates to a mass spectrometric apparatus for use with a liquid chromatograph, receiving a liquid sample ionized under an atmospheric condition.
With a mass spectrometric apparatus for use with a liquid chromatograph using an atmospheric pressure ionization (API) method such as an electro-spray ionization (ESI) method or an atmospheric pressure chemical ionization (APCI) method, a component separated by the liquid chromatograph is ionized under an atmospheric condition and introduced to a mass spectrograph (such as of the quadrupole type or the magnetic field type) kept under a high vacuum condition inside a chamber (hereinafter referred to as the high vacuum chamber). Because the degree of vacuum required inside such a high vacuum chamber is about 10.sup.-5 -10.sup.-6 torr and a nozzle must be provided to the chamber such that the ions of the liquid sample can pass therethrough to reach the mass spectrograph, it is difficult to directly connect the high vacuum chamber to the atmospheric environment. It is therefore a common practice to provide an interface comprising one or more chambers (hereinafter referred to as intermediate vacuum chambers) disposed between the atmospheric environment and the high vacuum chamber and to carry out multi-stage differential gas evacuation in two or more stages.
The degree of vacuum in these intermediate vacuum chambers is necessarily between those of the atmospheric environment and the high vacuum chamber. Hydrodynamically, the degree of vacuum may be divided into so-called viscous flow and molecular flow regions. By the viscous flow is meant a layer flow wherein collisions between gas molecules dominate, while it is a molecular flow if collisions between the gas molecules and the inner wall of the container are dominant. In many examples of a mass spectrometric apparatus of this type used with a liquid chromatograph having two intermediate vacuum chambers between the atmospheric environment in which the liquid sample is ionized and the high vacuum chamber containing the mass spectrograph, the degree of vacuum in the first intermediate vacuum chamber (on the upstream side closer to the liquid chromatograph) is about 1 torr and that in the second intermediate vacuum chamber (on the downstream side closer to the high vacuum chamber) is about 10.sup.-3 -10.sup.-4 torr, the degree of vacuum inside the high vacuum chamber being about 10.sup.-5 -10.sup.-6 torr. The degree of vacuum of about 1 torr inside the first intermediate vacuum chamber is within the viscous flow region. That of about 10.sup.-3 -10.sup.-4 torr inside the second intermediate vacuum chamber is in the region of intermediate and molecular flows.
It now goes without saying that nozzles must be provided to all partition walls through which ions of the liquid sample must travel from the atmospheric environment outside the chambers to the high vacuum chamber. Because these nozzles are each provided to a partition wall between two chambers which must be maintained at different degrees of vacuum, their openings must be very small, say, less than 1 mm in diameter. This makes it difficult to produce such apparatus with a high level of productivity especially because these small nozzle openings must be accurately aligned.
A method has been proposed whereby nozzles on the downstream side are intentionally positioned off the line of alignment such that only low-mass ions can pass through the nozzles on the downstream end and that charged particles with large masses and neutral particles (clusters) are blocked. By this method, however, the number of ions capable of passing through the nozzles on the downstream end decreases significantly, adversely affecting the detection sensitivity of the apparatus.